The applicant's long-term objective is to understand the pathophysiology of autosomal dominant polycystic kidney disease (ADPKD) as a basis for rational therapy, including gene therapy, of the disease. Recent studies have shed considerable light on the genetic basis of ADPKD. Mutations in PKD1 account for approximately 90 percent of cases. PKD1 encodes a cell-membrane protein, 'polycystin-1', that is most abundantly expressed in various tissues, including the kidney, during embryonic development. One of the most powerful approaches for the study of disease pathogenesis is targeted gene mutation. Standard transgenic technology results in insertion of the transgene at quasi-random sites in the genome so that the transgene and endogenous genes are simultaneously expressed. Gene targeting, by contrast, allows a mutant sequence to be substituted for the allelic endogenous sequence so that the mutation can be studied in the context of the natural chromosomal (regulatory) environment. Two different targeted mutants of Pkd1, the mouse homologue of PKD1, have recently been generated in the P.I.'s laboratory. Homozygotes develop severe polycystic kidney and pancreatic disease and die in the perinatal period. Heterozygotes develop scattered focal renal cysts throughout the kidneys and liver in a manner reminiscent of human ADPKD. The major goal of this proposal is to use these animals to identify the pathways that lead from polycystin-1 mutation to cyst formation. These animals will also be used to test existing hypotheses of cyst formation, including the role of apoptosis and the stimulation of EGF receptors abnormally localized on the apical surfaces of cyst lumens. Tissue- and stage-specific Pkd1-mutants will also be generated using the Cre-loxP system. Targeted mutant mice provide authentic models of human PKD1-disease that can be used to develop and evaluate new therapies for ADPKD. Such studies are much harder to perform in ADPKD patients because of the gradual progression of the disease in man and significant ethical and safety concerns.